Comment

Reaction of 1,2,3,5-tetramethylpyrazolium 4-toluenesulfonate with fuming nitric acid and concentrated sulfuric acid leads to nitration of both the cation and the anion. The salt isolated after quenching in ice and recrystallization from ethanol was found to be the title salt, (I).

Two previous reports give support to this nitration reaction. A kinetic study has indicated that the cation of 1,2,3,5-tetramethylpyrazolium bisulfate undergoes nitration to give the 1,2,3,5-tetramethyl-4-nitropyrazolium cation in a mixture of fuming nitric acid and concentrated sulfuric acid (Burton et al., 1971). In another study, 4-toluenesulfonyl chloride was reported to undergo nitration to give 4-methyl-3,5-dinitrobenzenesulfonic acid in a similar medium (Schmidt et al., 1999).

The existence of the dihydronium (H₅O₂⁺) unit was confirmed by the refinement. Initially, two O atoms (O1 and O2) were treated as water molecules and the associated H atoms were allowed to refine freely. On consideration of charge balance and the hydrogen-bonding scheme, and on examination of difference maps (visually using PLATON; Spek, 2003), it was believed likely that an additional H atom was located between the two water molecules [see, for example, Wells (1984) and Bernal & Fowler (1933)] but slightly closer to O2. Thus, an H atom was placed in this position and also refined freely, resulting in a position that was closer to O2, at a distance of 1.01 (5) Å. The group could be considered intermediate between clearly distinct (H₃O)⁺ and (H₂O) entities and an `(H₅O₂)⁺' entity, although it tends to be closer to the former. The O1O2 distance is 2.434 (3) Å, with an O-HO angle of 174 (5)°; this distance is significantly shorter than for other O-HO bonds within the structure (Table 1), again supporting the above interpretation.

Strong hydrogen bonds (Table 1) from the H₂OH₃O⁺ unit link to sulfonate O atoms. Atom O1 hydrogen bonds within the asymmetric unit, whereas O2 not only hydrogen bonds to O1, but also to symmetry-related ions. This gives rise to C(8) chains (Bernstein et al., 1995) along the [010] direction (Fig. 2), involving the three 4-methyl-3,5-dinitrobenzenesulfonate groups and the H₂OH₃O⁺ group.

Figure 1 The asymmetric unit of the title compound, showing part of the atom-labelling scheme and indicating how it continues for similar ions within the asymmetric unit. Displacement ellipsoids are drawn at the 50% probability level. H atoms have been omitted for clarity, except those in the (H5O2)+ unit, which are shown as circles of arbitrary radii.

Figure 2 Part of the crystal structure of the title compound, showing the formation of hydrogen-bonded C(8) chains along [010]. Atoms marked with an asterisk (*) or a prime (') are at the symmetry positions (-x, 1 - y, -z) and (x, y - 1, z), respectively. The two 1,2,3,5-tetramethyl-4-nitropyrazolium cations have been omitted for clarity, as have all H atoms, except those of the (H5O2)+ unit, which are shown as circles of arbitrary radii. Displacement ellipsoids are drawn at the 30% probability level.

Experimental

1,2,3,5-Tetramethylpyrazolium 4-toluenesulfonate was prepared from 1,3,5-trimethylpyrazine and methyl 4-toluenesulfonate following a similar procedure as used for 1,2,3,5-trimethyl-4-nitropyrazolium 4-toluenesulfonate (Burton, et al., 1971). Nitration of 1,2,3,5-tetramethylpyrazolium 4-toluenesulfonate was carried out under similar conditions used for 4-toluenesulfonyl chloride (Schmidt et al., 1999). The title compound was obtained by recrystallization from EtOH of the product isolated on careful addition of the reaction mixture to ice (m.p. 330-332 K).

Crystal data

2C7H12N3O2+·H5O2+·3C7H5N2O7S- Mr = 1161.00 Monoclinic, P 21 /c a = 26.0927 (5) Å b = 8.17230 (10) Å c = 24.0602 (6) Å = 106.7241 (8)° V = 4913.52 (17) Å3 Z = 4 Dx = 1.569 Mg m-3 Mo K radiation Cell parameters from 10634 reflections = 2.9-27.5° = 0.26 mm-1 T = 120 (2) K Slab, colourless 0.45 × 0.20 × 0.04 mm

Data collection

Bruker-Nonius KappaCCD diffractometer and scans Absorption correction: multi-scan(SADABS; Sheldrick, 2003)Tmin = 0.834, Tmax = 0.990 53726 measured reflections 11088 independent reflections 8214 reflections with I > 2(I) Rint = 0.044 max = 27.5° h = -33 33 k = -10 10 l = -29 31

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.050 wR(F2) = 0.145 S = 1.06 11088 reflections 732 parameters H atoms treated by a mixture of independent and constrained refinement w = 1/[2(Fo2) + (0.0891P)2 + 0.908P] where P = (Fo2 + 2Fc2)/3 (/)max = 0.001 max = 0.55 e Å-3 min = -0.59 e Å-3 Extinction correction: SHELXL97 Extinction coefficient: 0.0067 (5)

Table 1 Hydrogen-bond geometry (Å, °) D-HA D-H HA DA D-HA O2-H2CO1 1.02 (5) 1.42 (5) 2.433 (3) 176 (4) O1-H1AO541 0.83 (4) 1.85 (4) 2.681 (3) 176 (4) O1-H1BO341 0.90 (3) 1.79 (4) 2.690 (2) 173 (3) O2-H2AO441i 0.95 (3) 1.64 (4) 2.570 (2) 164 (3) O2-H2BO342ii 0.87 (3) 1.73 (3) 2.590 (2) 170 (3) C43-H43O462iii 0.95 2.46 3.401 (3) 172 C55-H55O342ii 0.95 2.55 3.212 (3) 127 Symmetry codes: (i) -x, -y+1, -z; (ii) x, y-1, z; (iii) x, y+1, z.

H atoms were located in difference maps and, except for those in the H₅O₂ group, were then treated as riding atoms, with C-H distances of 0.95 (aromatic) or 0.96 Å (methyl) and U_iso(H) values of 1.2U_eq(aromatic C) or 1.5U_eq(methyl C). The existence and location of the additional H atom in the H₅O₂ group was demonstrated from a difference map (see Comment); all H atoms of this group were allowed to refine freely.

Data collection: COLLECT (Hooft, 1998); cell refinement: DENZO (Otwinowski & Minor, 1997) and COLLECT; data reduction: DENZO and COLLECT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: OSCAIL (McArdle, 2003) and SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.